Holograms for reconstruction of objects of equal intensity



Unitecl States Patent Ofiicc 3,519,324 Patented July 7, 1970 U.S. Cl. 350-35 4 Claims ABSTRACT OF THE DISCLOSURE Equal intensity output images are obtained from each of a plurality of different holograms, all made with the same coherent light intensity and exposure time, even though the different objects from which the holograms are made may be of different size. This is accomplished by causing a fixed area of the coherent light beam crosssection to be intercepted during the preparation of the hologram, even though only a part of this area is occupied by the object, and upon playback, masking that portion of the reconstructed image not of interest.

BACKGROUND OF THE INVENTION In automated printing systems, a programmed digital computer selects from a memory the characters it is desired to print, causes these characters to be displayed on the screen of a cathode ray tube and a photograph taken of the displayed characters provides the basis for a printing master. Conventionally, the memory is a magnetic core memory or similar memory which stores binary information. However, investigations have indicated that price and other advantages may be possible it the printing fonts are stored optically in analog fashion. When the characters are stored in this way, they may be selected by illuminating the respective characters in response to addresses supplied by the computer. In one of a number of different ways, the successively illuminated characters readily may be translated into a permanent record and the latter may be employed to make a printing master.

More recently, it has been proposed that the fonts of characters be stored optically as holograms. However, as discussed in more detail below, it is found upon reconstruction of the holograms, that the intensity of the characters depends upon their size-the larger the area occupied by the character, the lower its intensity.

The object of this invention is to provide a method of preparing holograms in such a way that the reconstructed images of the holograms are of equal intensity, regardless of the size of the hologrammed object.

SUMMARY OF THE INVENTION During preparation, objects being holographed are made to exhibit the same apparent area relative to the coherent beam cross-section by placing additional material in the path of the beam. During readout, those portions of the reconstructed image containing the additional material are masked.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sketch showing a conventional way in which a hologram is made;

FIG. 2 is a sketch showing a way in which an image may be reconstructed from a hologram;

FIGS. 3-5 illustrate the way in which characters are prepared for holographing in accordance with the present invention;

FIG. 6 illustrates the way in which one object may be read out in accordance with the present invention; and

FIG. 7 is a block diagram of a portion of a system in accordance with the present invention.

DETAILED DESCRIPTION In the known arrangement of FIG. 1, light from a laser 10 is shined through a half-silvered mirror 12 onto a film 14. A portion of the light from the laser is retlected from the mirror 12 and mirror 16 through a diffuser 18. The object 20 being holographed is located between the diffuser and film and in the path of the laser.

beam. At the film, the two laser beams, one known as the reference" beam 21 and the other as the information beam 23, produce an interference pattern which is recorded on the film. The photographic record of this interference pattern is known as a hologram.

FIG. 2 illustrates how the hologram is read out. The developed film 14, with the interference pattern on it, is illuminated by laser 22. The latter may be at the same frequency as the laser 10 of FIG. 1 and, in fact, may be the same laser. The light from this laser strikes the hologram at the same angle as was employed to make the hologram. Perhaps 99% of the light energy passes directly through the film, as indciated by dotted line 24 and per-' haps 1% of the energy is employed to produce the reconstructed image shown at 26.

When making a large number of holograms of different objects, it is found convenient to use the same exposure time and same laser light intensity for these different objects. This is the case, for example, when the objects 20 consist of different alphanumeric characters and are being employed to make up a matrix of holograms on film 14 for use in an automated typesetting system of the type discussed briefly in the introductory portion of this application. However, it is found, in practice, that when made in this way the different holograms may produce images of different brightness when read out. The reason, according to one theory, is that the reconstructed images tend to be constructed of the same total energy. Upon read out in the manner illustrated in FIG. 2, a small object such as a period or comma or the like has a greater concentration of this energy and appears to be relatively bright, whereas a large object such as the letter A or E spreads this energy over a larger area and appears to be relatively dim. Of course, in applications such as those discussed herein, this is a great disadvantage because in the printing masters made up from the reconstructed holograms, the characters will not be uniformly black.

A hologram storage plate for a large number of alphanumeric characters may be made by successively placing the character masks at position 20 in FIG. 1, and for each such mask moving the film to a position such that the reference beam 21 intersects the film at a desired location and blocking (preventing light from reaching) all areas of the film 14 except that at which the hologram of the character is to be recorded. After all characters are recorded in this manner and the film 14 is developed, the film, which now may be termed a storage plate, consists of a number of small holograms, one for each of the characters. Each character mask, according to the invention, consists of two squares, one within the other, as shown in FIGS. 3-6. For all characters, each pair of such areas is the same size. Within the smaller square shown at 30 in FIG. 3, is placed the character it is desired to holograph.

The character may be opaque and the background transparent. Or, if the hologram is to be made by reflected light, the character may be black and the background white. Also, in each case, the reverse is possible, that is transparent characters on an opaque background or white characters on a black background. In the discussion which follows, the characters, shown crosshatched, are assumed to be opaque and on a transparent background.

In FIG. 3, the character illustrated is the letter A. This character only occupies a portion of the square 30. An area equal to the remaining portion of the square 30 is made opaque and placed in the region between the square 30 and the larger square 32. In FIG. 3, this is shown to consist of five regions 34-38. The placement of the areas 34-38 is not critical nor is the shape of these areas. All that is necessary is that the sum of the opaque area occupied by the character Within the square 30 and the opaque areas outside of the square 30 be equal to the area of the square 30.

FIGS. 4 and 5, respectively, illustrate the principle above as applied to the letter I and to a punctuation mark, namely a period. The remaining characters of the font which it is desired to store on the hologram storage plate are prepared in exactly the same way.

The way in which a hologram may be read out in accordance with the present invention is illustrated in FIGS. 2 and 6. When the hologram 14 of FIG. 2 is illuminated by the beam from the laser 22, an image of the holographed object appears at 26. When the hologram 14 consists of a number of small holograms, each of a different character, and the laser beam is directed to strike any one of the small holograms, the image corresponding to that hologram will appear at 26. When the laser beam is directed to strike any other of the small holograms, other characters, corresponding to the other holograms, will appear at 26.

In the present invention, position 26 of FIG. 2 contains a masking element formed with a square opening as indicated by 42 in FIG. 6. Referring to FIG. 6, when the laser beam 44 strikes hologram 40, an image of the character appears at 46. Those parts of the character in the inner squares 30 of FIGS. 3, 4 and will be projected through the mask 42 of FIG. 6. Those parts of the characters outside the small squares 30 of FIGS. 3, 4 and 5 will be intercepted by the mask 42 of FIG. 6. Thus, when viewed from the right side of the mask 42 of FIG. 6, only the desired portions of the character images will be seen. As explained earlier, each of the characters will be of the same brightness regardless of the area of the image.

A system according to the present invention is shown in FIG. 7. It includes a laser 50, a deflection system 52 and a hologram storage plate 54 made u in the way discussed above. This storage plate may store 100 or more holograms. The system also includes a mask 56 formed with a single square opening therein and corresponding to the mask 42, shown in FIG. 6. A vidicon camera 58 is located beyond the mask 56.

In operation, an input address from a computer (not shown) is applied to the digital-to-analog converter 60. The analog voltages produced by the converter are applied to the deflection system 52 and the latter deflects the laser beam to a desired location on the hologram storage plate 54. The character produced by the illuminated hologram, regardless of the position of the hologram on the storage plate 52, is projected through the mask 56 and onto a common location on the vidicon of the camera 58. This is so because two conditions are met. First, the readout beam 62 is in exactly the same position as the reference beam 21 of FIG. 1 was in when exposing the hologram. Second, the vidicon of camera 58 is in a position conjugate to that of the object 20 (the character mask from which the hologram was made) in FIG. 1. The square aperture in the mask 56 (FIG. 7) through which the projected image passes, prevents the undesired portions of the image, namely those corresponding to 3438 of FIG. 3, from reaching the vidicon.

When any image is projected onto the vidicon target, the target can be scanned by any of the well-known deflection schemes to generate a video signal which, in conjunction with the scanning waveforms, describe the character projected Onto the vidicon. The video and deflection waveforms may be applied to a cathode ray tube (not shown) to cause the electron beam thereof to trace out the character on its phosphor screen. When a number of such characters are written side by side and in sequence, a line of type is presented on the screen of the cathode ray tube. Different characters to be written may be selected by directing the laser beam to different areas of the hologram storage plate 54 of FIG. 7.

The various components shown in the system of FIG. 7, aside from the storage plate and mask are, in themselves, well-known. The electronic deflection system, for example, may be of the type generally discussed in the article A Fast, Digital-Indexed Light Deflection," by W. Kulke, et al., IBM Journal, vol. 8, page 64, 1964.

To make a hologram storage plate such as discussed herein, the various character masks including those shown in FIGS. 3-5, may be stored on a successively advanced film strip and the holograms of these masks positioned at desired locations on the plate by mechanically moving the plate. Details of a technique of this kind are given in application Ser. No. 515,531, filed Dec. 22, 1965 by R. S. Mezrich et al. and assigned to the same assignee as the present application.

What is claimed is:

1. A method of holographing objects of different size comprising the steps, during the preparation of the holograms, of:

illuminating each object with a coherent light beam to provide the information component required to make the hologram of that object, the object, in each case, occupying a certain area of the coherent beam crosssection;

placing in the same coherent light beam a suflicient amount of additional material of substantially the same opacity as the object so that the sum of the area of the coherent light beam cross-section it occupies plus the sum of the coherent light beam area occupied by the objects is in each case equal to the same value; and

for each object, using the same coherent light beam intensity and exposure time.

2. The method of claim 1 wherein the object is a character located in a first region and the additional material is located in a second region surrounding the first region.

3. A method of holographing characters comprising the steps, during the preparation of the holograms, of:

placing each character within a first area of given size;

placing each first area within a second area of given size;

placing in each region of the second area outside of the first area, markings of the same opacity as the character and occupying an area such that the sum thereof plus that of the character, in each case is equal to said first area;

illuminating each second area with a coherent light beam to provide the information component required to make the hologram of everything within each second area; and

for each second area, using the same coherent light beam intensity and exposure time to make the hologram of that area; and

the steps, during the reconstruction of the image of each hologram, of:

shining coherent light through the hologram; and

masking the region of the reconstructed image corresponding to the region of the second area outside of the first area, whereby only the image of the character within a first area is visible, in each case.

4. In the preparation of holograms of characters, the improvement comprising the masters from which the dif ferent holograms are made all being of the same size and all with the same ratio of opaque to transparent area, each such master comprising one region within which the character is located and second region Within which other material of the same opacity as the character is located, the sum of the area occupied by the character in the one 6 region and the area occupied by the other material in the OTHER REFERENCES second region, in each case being equal to the area of the first region.

References Cited UNITED STATES PATENTS 2,770,166 11/1956 Gabor 3503.5

DAVID SCHONBERG, Primary Examiner 5 R. L. SHERMAN, Assistant Examiner Graphic Technology, pp. 7-11, January 1967. 

